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Cosa wrote:
While trying to run kvik's lu9 on ARM, I found that when converting
LLONG_MIN to a double on 32bit systems, the result is wrong (positive
instead of negative).
Given the following test program:
void main() {
vlong min = LLONG_MIN;
double dmin = min;
print("minint %lld\n", min);
print("minint as double %f\n", dmin);
if (dmin > 0.0) {
exits("int min as double turned positive");
}
exits(0);
}
The output on x86_64 will be:
minint -9223372036854775808
minint as double -9223372036854776400.000000
But on arm or 386 (and I expect also spim, 68000, mips, 68020, sparc,
power, since they all use the same _v2d):
minint -9223372036854775808
minint as double 9223372036854776400.000000
And the value turned positive in the conversion.
The function used for the cast to double is (in /sys/src/libc/arm/vlrt.c):
double
_v2d(Vlong x)
{
if(x.hi & SIGN(32)) {
if(x.lo) {
x.lo = -x.lo;
x.hi = ~x.hi;
} else
x.hi = -x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
}
If I understand correctly, the issue is that where it tries to flip the
sign for x.hi (x.hi = -x.hi), 0x80000000 has no positive, thus stays the
same (it stays negative). Then when we get to the negative return, we
get a positive out.
What came to my mind then, is that in the case that there is no x.lo, we
can keep the x.hi sign and cast directly, thus:
double
_v2d(Vlong x)
{
if(!x.lo) {
return (long)x.hi*4294967296.;
}
if(x.hi & SIGN(32)) {
x.lo = -x.lo;
x.hi = ~x.hi;
return -((long)x.hi*4294967296. + x.lo);
}
return (long)x.hi*4294967296. + x.lo;
}
This looks correct to me, but I don't trust myself to not make mistakes
in such critical code, so I would like some feedback on the change.
Happy new year in advance,
cosa
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amd64 passes first argument in RARG (BP) register
which has the be preserved duing _profin() and
_profout() calls. to handle this we introduce
_saveret() and _savearg(). _saveret() returns
AX, _savearg() returns RARG (BP). for archs other
and amd64, _saveret() and _savearg() are the
same function, doing nothing.
restoing works with dummy function:
uintptr
_restore(uintptr, uintptr ret)
{
return ret;
}
...
ret = _saveret();
arg = _savearg();
...
return _restore(arg, ret);
as we pass arg as the first argument, RARG (BP) is
restored.
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